The influence of graphene platelets (GPLs) on the WC grain size of WC-Co-GPLs cemented carbide prepared by low-pressure sintering was investigated. The role of GPLs in refining WC grains was explored by characterizing grain size and phase distribution. Results show that the addition of GPLs leads to significant grain refinement of WC and the more uniform distribution of WC grain size. When the content of GPLs is 0.10wt%, the average WC grain size in the cemented carbide is 0.39 μm, which is 32% lower than that in WC-Co. However, the shape of WC grains is almost unaffected, while the mean free path of Co decreases. The grain refinement of WC is attributed to the homogeneous distribution of GPLs between WC/WC and WC/Co grain boundaries, which hinders the solution and precipitation process of WC in liquid phase Co, as well as the migration and growth of WC grains. Additionally, GPLs can serve as heat transfer plates in materials to improve cooling efficiency, thus inhibiting the growth of WC grain.